In the packaging industry, use has long been made of packages of a single-use nature (so-called single-use disposable packages) for packing and transporting products such as, for example, liquid foods. A very large group of these single-use packages is produced from a laminated packaging material comprising a core layer of paper or paperboard and outer, liquid-tight coatings of plastic, preferably polyethylene, on both sides of the core layer.
The composition of a packaging material is intended to impart to the package the best possible product protection properties for the packed product, at the same time as making the package both easy to produce and easy to handle. A core layer gives the package good mechanical configurational stability so that the package may be distributed in a simple, rational manner, and be conveniently handled, at the same time as outer, liquid-tight coatings of plastic afford effective protection against the penetration of moisture and wet into the packaging material. Preferably, the outer, liquid-tight plastic coatings consist of a thermoplastic, for example polyethylene, which moreover imparts to the laminated packaging material excellent thermosealing properties such that the packages may permanently be given their desired geometric configuration by conventional thermosealing during the conversion of the laminated packaging material into finished packaging containers.
Such packaging containers are often produced in that a web of packaging material is reformed into a tube by the longitudinal edges of the web being united to one another, whereafter the tube is filled with the intended contents and is sealed along narrow, transverse, spaced-apart sealing zones. The sealed-off portions of the tube thus containing their intended contents are then separated from the tube by means of incisions in the above-mentioned-sealing zones and are formed by folding into optional geometric configurations depending on the orientation of the sealing joints or seams.
A laminated packaging material comprising a core layer and outer plastic coatings as described above thus makes for a rational production of configurationally stable, liquid-tight packages possessing superior tightness properties against moisture and wet, but almost entirely lacks tightness properties vis-a-vis gases, in particular oxygen gas.
In order to supplement the prior art laminated packaging material with such gas tightness properties, it is known in the art to apply a layer of metal or metal oxide, for example aluminum (Alifoil) on one side of the core layer between the core layer and the one outer plastic coating of the packaging material.
A feature common to the above-described packaging containers is that an incision edge of the packaging material will be exposed to the contents of the finished package if the tube joint or seam is of the overlap type, which in turn implies that the contents may readily be sucked into the packaging material if this includes an absorbent material such as, for example, paper or paperboard. If the contents are sterilized and packed under aseptic conditions, the unprotected incision edge must not come into contact with the contents, but must be protected in some way or other.
In many cases, the packaging material web undergoes a sterilization stage before being formed into a tube, for example by dipping in a bath of hydrogen peroxide solution. If the core layer includes a fibrous, absorbent material, the further drawback arises that the packaging material sucks up sterilization liquid which may cause difficulties in the longitudinal joint seal in the form of, for example the occurrence of vapor blisters. It is therefore desirable also from this viewpoint to protect the incision edge against the penetration of liquid and moisture.
The most important reason why the incision edge must be screened off from the packed product when the packaging material includes a metal layer is that metals, in compliance with laws and regulations on packaging of foods, may not be in direct contact with the food products because of the negative effects that metals have we on such products.
One known method of avoiding direct contact between the incision edge and the contents of the package is to cover the incision edge with a longitudinal plastic strip which, at the same time as the tube is formed, is advanced and sealed in place against the inner plastic coating of the packaging material tube over the incision edge. This method is circumstantial to put into effect, since the loose strip must be positioned in connection with the longitudinal joint sealing operation.
A further method of solving the problem of'sealing/tightening the incision edge of the packaging material is, on production of the packaging material, to give the inner plastic layer such width that it projects out with an edge zone over the one edge of the core or carrier layer, in which event the projecting edge zone is, on production of the packaging material tube, sealed against the inner plastic coating of the opposing, overlapping edge zone, in such a manner that the incision edge is automatically sealed off.
Swedish Patent SE 78121423 describes how the incision edge of the longitudinal joint on the inside of a tube of a packaging material including a layer of aluminium is covered by a projecting aluminium foil strip and a plastic strip which projects further out from the core layer and which is sealed to the inner plastic coating of the opposing edge zone. Since the aluminium foil strip also covers the incision edge, vapor or steam blisters are prevented from occurring on sealing of the longitudinal joint when the water which is always bonded in the fibrous core or carrier layer is vaporized.
However, it has proved to involve certain difficulties to seal such a thin, projecting plastic strip to the inner plastic layer of the opposing edge, on the one hand because it is difficult to keep the projecting strip in place during the sealing operation and, on the other hand, because it is difficult to control the sealing tool such that the sealing pressure is applied equally and simultaneously over both the free plastic strip and over the overlap region proper in the longitudinal joint zone.
United Kingdom Patent GB 1013656 describes a method of folding over such a projecting plastic strip as described above around the edge of the overlapping laminate and sealing it to the plastic-coated inside of the subjacent, opposing edge zone. In such a fold-over operation, it is also difficult to position the projecting edge strip so that the longitudinal joint is protected and tight throughout the entire length of the packaging material tube in an operationally reliable manner.
A feature common to the various methods of production including a plastic strip projecting from the web edge is that the lamination process becomes circumstantial and complicated when the packaging material is produced. Either the layer in which the projecting strip is included must be applied such that it extends outside the edges of the rest of the laminate web, or it must be first applied and then, in a second stage, be delaminated and released from the rest of the laminate web in the edge zone regions.
The above-described prior art technology is based on the assumption that the one edge of the packaging material overlaps the opposing edge so that the longitudinal joint of the material tube is strong and durable. However, the drawback then arises that the wall of the packaging container will be twice as thick in the longitudinal joint area than in the rest of the package.